1. Field of the Invention
The present invention relates to a spark plug used for providing ignition of an internal combustion engine.
2. Description of the Related Art
In recent years, in order to improve the performance of an internal combustion engine such as an automobile engine, or to cope with tightened emission gas regulations or to enhance combustion efficiency, the engine has employed lean burn, which is accompanied by a tendency toward an increase in the electrode temperature of a spark plug used for providing ignition of the engine. Particularly, a ground electrode exhibits greater temperature rise than does a center electrode, since the ground electrode is located deeper in a combustion chamber. Particularly, in the case of a spark plug for use in a direct-injection-type engine or the like, the ground electrode is more likely to exhibit marked temperature rise. Under the above-mentioned severe conditions, spark ablation of an electrode tends to be accelerated. In order to suppress the expansion rate of a spark discharge gap, a spark plug having a noble metal chip welded to a ground electrode at a portion facing a spark discharge gap has been widely used.
An increase in the temperature of a ground electrode raises a problem of high-temperature oxidation of an electrode base metal, to which a noble metal chip is welded. Conventionally, in order to attain high-temperature oxidation resistance, an Ni-based heat resistant alloy such as INCONEL 600 (INCONEL is the trade name of a product from available INCO Corp., UK) has often been used as a base metal of the ground electrode. However, the thermal conductivity of an Ni-based heat resistant alloy is generally not very high; thus, the Ni-based heat resistant alloy exhibits poor heat release and raises a problem of exhibiting a tendency toward a high rise in electrode temperature particularly in high-speed operation or the like. A rise in electrode temperature resulting from poor thermal release leads to a rise in the temperature of a metal chip joined to the electrode base metal, thereby shortening the life of the metal chip through abnormal ablation. In order to accelerate thermal release, a method has been proposed for suppressing a temperature rise of an electrode by means of disposing a core formed from a Cu-based metal (a Cu-based heat transfer acceleration element) in an electrode base metal (e.g., Japanese Patent Application Laid-Open (kokai) No. H05-159857 and Japanese Patent Publication (kokoku) No. H06-48629).
2. Problems Solved by the Invention
However, a further increase in combustion temperature and further approach of a spark portion to the center of a combustion chamber as in the case of the above-mentioned direct-injection-type engine involve a more significant increase in the temperature of a ground electrode. As a result, INCONEL 600 or a like alloy used as an electrode base metal fails to sufficiently resist high-temperature oxidation. In this case, the electrode base metal may be replaced with a metal having higher high-temperature oxidation resistance. For example, replacement of conventionally used INCONEL 600 with INCONEL 601 has been proposed. INCONEL 601 has higher Cr and Fe contents and therefore exhibits enhanced high-temperature oxidation resistance. However, such replacement of materials raises a significant problem when embedment of a Cu-based heat transfer acceleration element is to be employed.
Specifically, an electrode having a Cu-based heat transfer acceleration element is formed in the following manner: a Cu material which is to serve as the Cu-based heat transfer acceleration element is embedded in an Ni alloy material which is to serve as an electrode base metal, thereby yielding an assembly; and the assembly is subjected to cold working such as drawing, forging, or rolling, thereby yielding a clad wire material. However, a nickel-based heat resistant alloy of increased Cr content, such as INCONEL 601, exhibits high deformation resistance and low ductility as compared with INCONEL 600 or the like, as is commonly observed with a metal material whose strength is enhanced by increasing the alloying element content. Therefore, the above-mentioned process for manufacturing a clad wire material having the Cu-based heat transfer acceleration element is apt to involve cracking or a like problem, thereby raising a problem of a great reduction in yield. When a spark plug in which the Cu-based heat transfer acceleration element is embedded in the electrode base metal formed predominantly from Ni is used in an engine, a diffusion layer is formed such that metal components are diffused between the electrode base metal and the Cu-based heat transfer acceleration element. As a result of being subjected to repeated load stemming from the thermal expansion difference between the electrode base metal and the Cu-based heat transfer acceleration element, separation may arise in the diffusion layer. As a result, heat may fail to be sufficiently conducted from the electrode base metal to the Cu-based heat transfer acceleration element. When the Cu-based heat transfer acceleration element is eliminated, high-temperature oxidation of the electrode base metal can be suppressed, but the temperature rise of a noble metal chip cannot be suppressed. Thus, a problem of abnormal ablation of the chip cannot be solved.
An object of the present invention is to provide a spark plug in which sufficient high-temperature oxidation resistance is imparted to an electrode base metal of a ground electrode. The subject ground electrode has a structure including an embedded Cu-based heat transfer acceleration element exhibiting better thermal conductivity than that of the electrode base metal and is adapted to suppress a temperature rise of the electrode, the structure being able to be formed through cold working without encountering problems associated with cold working, and in which abnormal ablation of a noble metal chip joined to the electrode base metal can be prevented.
The above object of the present invention has been achieved by providing a spark plug comprising a tubular metallic shell (1), an insulator (2) fitted into the metallic shell (1), a center electrode (3) provided in the insulator (2), and a ground electrode (4), one end of the ground electrode (4) being joined to the metallic shell (1) by means of welding or a like process, and a spark discharge gap (g) being formed between the other end portion of the ground electrode (4) and the center electrode (3). The spark plug is further characterized in that the ground electrode (4) comprises an electrode base metal (4a), a heat transfer acceleration element (4c) embedded in the electrode base metal (4a), formed predominantly from, for example, Cu, and exhibiting higher thermal conductivity than that of the electrode base metal (4a), and a noble metal chip (32) welded to the electrode base metal (4a) at a position facing the spark discharge gap (g). The electrode base metal (4a) comprises an Ni alloy containing Cr in an amount of 14%-17% by mass, Mo in an amount of 0.8%-3.5% by mass, and Ni in an amount of 68%-85.2% by mass. Herein, the term xe2x80x9cpredominantxe2x80x9d or xe2x80x9cpredominantlyxe2x80x9d used in relation to content means that the subject component is present in the highest content by mass.
In the above-described spark plug of the present invention, the Cu-based heat transfer acceleration element is embedded in the electrode base metal of the ground electrode so as to accelerate heat release, thereby suppressing temperature rise and thus extending the life of the ground electrode. Also, since the temperature rise of the noble metal chip welded to the electrode base metal is suppressed, abnormal ablation of the noble metal chip is prevented, thereby ensuring durability. The present invention employs an Ni alloy of the above-mentioned composition as the electrode base metal, thereby yielding the advantage described below as compared with the case of employing INCONEL 601 or the like as practiced conventionally and without encountering the above described problems of the prior art. When an Ni alloy containing C is to be employed as in the case of the present invention, addition of a certain amount of Mo together with Cr greatly enhances the high-temperature oxidation resistance of the alloy. Therefore, by virtue of employing the composition in combination with the Cu-based heat transfer acceleration element, even when the spark plug is used under severe conditions, the ground electrode can maintain sufficient durability and thus can exhibit extended life.
In this case, particularly in the case of an Ni alloy containing C, addition of Mo yields an effect of improving high-temperature corrosion resistance. The carbon may be contained as an impurity or may be intentionally added so as to enhance precipitation in the form of carbide (a so-called weak-precipitation alloy). The C content is adjusted to not greater than 0.3% by mass. Particularly, in the latter case, the C content is adjusted to, for example, 0.03%-0.3% by mass. However, when the C content is excessively high, a large amount of carbide is formed, thereby impairing cold workability. Therefore, the C content is preferably not higher than 0.10% by mass. In either case, when Mo is not added, contained C forms a carbide mainly with Cr. When such a Cr carbide is formed in a large amount, the amount of Cr, which is an element for imparting oxidation resistance, decreases as a result from precipitation of Cr in the form of Cr carbide. As a result, a passivation oxide film is insufficiently formed, leading to impairment in oxidation resistance. Particularly, when a Cr carbide is formed at a grain boundary, a Cr-deficient layer is formed in the vicinity of the grain boundary. Such formation leads to a tendency toward intergranular corrosion while the local-cell effect reinforces the development of the tendency, thereby further exerting an adverse effect on the durability of the electrode base metal.
However, when Mo is added in an appropriate amount, an Mo carbide is formed in precedence to a Cr carbide, thereby suppressing precipitation of a Cr carbide and increasing the amount of Cr contributing to formation of a passivation oxide film. As a result, even in the case where Cr content is fixed, a stronger passivation oxide film can be formed, thereby contributing to enhancement of high-temperature corrosion resistance. Also, since an Mo carbide is generally unlikely to precipitate at a grain boundary, a Cr-deficient layer is unlikely to be formed. Thus, an Mo carbide acts advantageously to suppress intergranular corrosion.
As a result, even when the Cr content is set to a relatively low level of 14%-17% by mass, the above-mentioned effect of addition of Mo implements high-temperature corrosion resistance equivalent to or higher than that exhibited by INCONEL 601 or a like alloy, which contains Cr in a higher amount. Therefore, since cold workability is improved to a degree corresponding to a reduction in the Cr content, a clad material in which a Cu-based heat transfer acceleration element is embedded and from which a ground electrode is formed can be manufactured without problem.
Even in long-hour use in an engine, the addition of Mo can yield the effect of suppressing an increase in the thickness of a diffusion layer formed in the boundary between the electrode base metal and the Cu-based heat transfer acceleration element, thereby preventing separation in the diffusion layer. Conceivably, low ductility of an alloy of Ni and Cu contained predominantly in the diffusion layer may be related to the occurrence of the separation.
When the Cr content of an Ni alloy serving as the electrode base metal is less than 14% by mass, the high-temperature oxidation resistance of the electrode base metal becomes insufficient, thereby shortening electrode life. When the Cr content is in excess of 17% by mass, workability is impaired, resulting in a tendency toward the occurrence of cracking or the like in the course of manufacturing a clad material in which a Cu-based heat transfer acceleration element is embedded and from which a ground electrode is formed.
When the Mo content is less than 0.8% by mass, addition of Mo poorly yields an effect of improving high-temperature oxidation resistance and an effect of preventing separation in the diffusion layer in long-hour use. When the Mo content is in excess of 3.5% by mass, the hardness of a resultant alloy increases, thereby increasing deformation resistance and thus leading to impaired workability. When the Ni content is less than 68% by mass, the accessory-component content becomes excessively high, resulting in a tendency toward impaired workability or the like. When the Ni content is in excess of 85.2% by mass, the required Cr and Mo contents cannot be attained, thereby leading to impaired high-temperature oxidation resistance.
In view of ensuring weldability, or weld strength, in welding a ground electrode to a metallic shell, preferably, an Ni alloy serving as the electrode base metal has an Al content less than 1% by mass. When the Al content is not less than 1% by mass, aluminum oxide is excessively formed, thereby potentially impairing weldability or weld strength. For the purpose of enhancing high-temperature oxidation resistance, Al can be intentionally added within the above-mentioned range.
Fe can be added to an Ni alloy serving as the electrode base metal. Fe forms a solid solution containing Fe and Ni in order to increase the strength of the alloy to thereby enhance its high-temperature strength. Preferably, the Fe content is adjusted to 6%-10% by mass. When the Fe content is less than 6% by mass, the contained Fe falls to yield a sufficient effect of enhancing high-temperature strength. When the Fe content is in excess of 10% by mass, high-temperature oxidation resistance may fail to be sufficiently attained.